The present invention relates to systems for sensing a fluid. More particularly, embodiments of the invention relate to mechanisms and techniques for reducing interference in measurements caused by air bubbles (e.g., a gas trapped in a liquid) in fluid level and concentration sensors.
Fluid level and fluid concentration sensing is important in a number of vehicle applications including, for example, the sensing of Diesel Exhaust Fluid (DEF) used in a selective catalytic reduction diesel emission-control system. Selective catalytic reduction (SCR) is a method of converting diesel oxides of nitrogen (NOx) emissions, by catalytic reaction, into diatomic benign nitrogen gas (N2) and water (H2O). DEF is used in the process. In clean diesel engines, an SCR system delivers near-zero emissions of NOx.
DEF is a mixture of purified water and urea. In a typical SCR system, DEF is stored in a tank of a vehicle and is injected via one or more injectors into the exhaust at a ratio of about 1:50 to the diesel fuel being burned. The injected urea (in the form of a mist) mixes with the exhaust and breaks down NOx in the exhaust into nitrogen, water, and carbon dioxide.
When contaminants such as diesel fuel, water, and ethylene glycol, mix with the DEF, the ability of the DEF to reduce the NOx in the exhaust is diminished. Contaminated DEF may also cause damage to the NOx reluctant system. It is also important that a sufficient amount of DEF be available for use in the SCR system. In or near the tank, one or more sensors are used to sense certain characteristics of the DEF. The sensors may include, but are not limited to: a level sensor for determining a quantity of DEF in the tank; a concentration sensor for determine the quality of DEF in the tank; and a temperature sensor. Fluid level is representative of the amount or quantity of fluid and concentration is one characteristic that is representative of the quality of the fluid.